Aluminum-zinc alloys having ancillary additions of lithium

ABSTRACT

An aluminum-copper-zinc alloy having ancillary additions of lithium. The alloy composition includes from about 5 to 13 wt % zinc and from about 0.01 to 1.0 wt % lithium.

BACKGROUND OF THE INVENTION

[0001] This invention relates to aluminum-zinc alloys having ancillary additions of lithium in order to decrease density while at the same time increasing the strength of the aluminum-zinc alloy and maintaining the resistance to corrosion.

[0002] In the aircraft industry, it is well known that one of the most effective ways to reduce the weight of an aircraft is to reduce the density of aluminum alloys used in the aircraft construction. For purposes of reducing the alloy density, lithium additions have been made. Molten lithium, however, is a highly reactive and highly aggressive material, which is difficult to handle and which is also difficult to alloy with the base alloy. Because of its high reactivity, any moisture in the presence of the molten aluminum-lithium can cause explosions. In addition, because of its highly aggressive nature, special refractories must be used in casting.

[0003] What is still needed is an aluminum alloy useful for, among other things, aerospace applications which not only has low density, high strength and good corrosion resistance, but also good fracture toughness.

SUMMARY OF THE INVENTION

[0004] The aluminum alloy of the invention has met or exceeded the above-mentioned needs as well as others. The aluminum alloy comprises from about 5% to 13% zinc and from about 0.10 to 0.99% lithium. It has been found, quite surprisingly and unexpectedly, that the ancillary additions of low levels of lithium to aluminum-zinc alloys provided a high strength, low density material that exhibits good fracture toughness and corrosion resistance over aluminum-zinc alloys without lithium additions and those aluminum-zinc alloys having lithium additions above 1.0 wt %.

BRIEF DESCRIPTION OF THE DRAWINGS

[0005] A full understanding of the invention can be gained from the following detailed description of the invention when read in conjunction with the accompanying drawings in which:

[0006]FIG. 1 is a chart showing the tensile yield strength of various specimens made from aluminum-zinc alloys designated Alloy A, Alloy B, Alloy C and Alloy D after being subjected to different aging conditions.

[0007]FIG. 2 is a bar graph showing the improvement in specific strength for some of the specimens shown in FIG. 1.

[0008]FIG. 3 is a plot of strength versus fracture toughness comparing a conventional aluminum alloy (AA 7150) to the alloys of the invention.

[0009]FIG. 4 is a table showing the results of the exfoliation corrosion testing for the alloys mentioned in the EXAMPLE.

[0010]FIGS. 5A and 5B are Dark Fields (DF) Transmission Electron Micrographs from samples from alloys A and C, respectively.

DETAILED DESCRIPTION

[0011] For the description of alloy compositions that follow, all references are to weight percentages (wt %) unless otherwise indicated. When referring to any numerical range of values, such ranges are to be understood to include each and every number and/or fraction between the stated range minimum and maximum. A range of about 0.01 to 0.99 wt % lithium, for example, would include all intermediate values of about 0.02, 0.03, 0.04 and 0.1 wt % all the way up to and including 0.97, 0.98 and 0.9895 wt % lithium. The same applies to the other elemental ranges set forth below. The term “substantially free” means having no significant amount of that component purposely added to the alloy composition, it being understood that trace amounts of incidental elements and/or impurities may find their way into a desired end product.

[0012] The present invention relates to an aluminum-zinc alloy having ancillary additions of lithium. In accordance with the invention, a wrought aluminum-zinc-lithium alloy is provided which has less density, improved strength and fracture toughness and comparable corrosion resistance to aluminum-zinc alloys without ancillary additions of lithium. The alloys of the present invention can be fabricated into plate, extruded or forged products. From these products, integrally stiffened structural parts can be machined. These parts can provide lower manufacturing costs than built-up structures and also provide greater design flexibility to the development of aerospace and space structural components. Examples of aerospace structural components are stringers, wing spars and upper wing sections, among others. These alloys provide high strength and low density while at the same time providing increased toughness and, surprisingly, no degradation in corrosion resistance over aluminum-zinc alloys having no lithium additions.

[0013] The alloys of the present invention can also be used for recreational products, such as baseball and softball bats, arrow shafts, golf club shafts and tubing for bicycles. The alloys of the invention have improved specific properties and modulus of elasticity thus resulting in recreational products having improved performance.

[0014] In accordance with the invention, the alloy of the invention has good strength and good fracture toughness. The yield strength (L) of the alloys of the invention are preferably above about 80 ksi and more preferably above about 85 ksi. The fracture toughness of the alloys of the invention are preferably above about 25 ksi {square root}{square root over (inch)}, more preferably above about 33 ksi {square root}{square root over (inch)} and most preferably above about 35 ksi {square root}{square root over (inch)}. The alloy of the invention will also preferably have a combination of (i) good strength and (ii) fracture toughness of preferably (i) above 80 ksi and (ii) above 30 ksi {square root}{square root over (inch)}. Finally, the alloy of the invention will have good corrosion resistance, as measured by the ANCIT test, which will be explained below.

[0015] The compositional ranges of the main alloying elements (zinc, copper, magnesium and lithium) of the improved alloy of the invention are broadly defined as follows: (1) from about 5 to 13 wt % zinc; (2) from about 1 to 3 wt % copper; (3) from about 1 to 6 wt % magnesium; and (4) from about 0.10 to 0.99 wt % lithium. The balance of the aluminum alloy of the invention contains aluminum and incidental impurities.

[0016] In addition to aluminum, zinc, copper, magnesium and lithium, the alloys of the present invention can contain alloying elements which form dispersoids selected from the group consisting of chromium, vanadium, titanium and zirconium and mixtures thereof in the range of from about 0.0 to 0.6 wt % and/or other elements which form dispersoids such as manganese, nickel, iron, hafnium and scandium and mixtures thereof in the range of 0 to 1 wt %. [Other alloying elements, such as silver, silicon and indium and mixtures thereof in amounts up to about 1.0 wt % can also be added.] The zinc in the alloy is added to increase the strength of the alloy. High amounts of zinc can be added as this element exhibits a large solid solubility in aluminum at intermediate temperatures. Care should be taken not to exceed maximum solid solubility as this can lead to low fracture toughness and low damage tolerance.

[0017] The copper is added to increase the strength of the aluminum base alloy and its resistance to stress-corrosion cracking and exfoliation corrosion. Copper additions beyond maximum solubility can lead to low fracture toughness and low damage tolerance.

[0018] The magnesium is added to provide strength and reduce density. Care should be taken, however, to not add too much magnesium since magnesium additions beyond maximum solubility will lead to low fracture toughness and low damage tolerance.

[0019] The lithium is added to reduce density and to increase strength. Care should be taken, however, in not adding too much lithium since exceeding the maximum solubility will lead to low fracture toughness and low damage tolerance. Lithium additions in amounts of about 1.5 wt % and above result in the formation of the δ′ (“delta prime”) phase with composition of Al₃Li. The presence of this phase, Al₃Li, is to be avoided in the alloys of the present invention.

[0020] The interaction of lithium atoms in supersaturated solid solution, with atoms of magnesium and/or copper appear to give rise to the formation of clusters of atoms of solute. These clusters of solute act as seeds for the nucleation of strengthening precipitates. This results in a larger number of precipitates in the alloys containing the lithium additions. Furthermore, a larger number of nucleation sites leads to smaller precipitate size. Finally, as the supersaturation from solid solution is released, the driving force for the heterogeneous nucleation of deleterious precipitates at the grain boundaries is reduced. Without limiting the invention, it is believed that this may contribute to the high fracture toughness of the lithium containing alloys.

[0021] It should be noted that zinc, copper and magnesium in the compositional ranges set forth above will be soluble only in appropriate mixtures as defined by the equilibrium phase diagram. The ancillary additions of lithium reduce the maximum solubility of all alloying elements. There is no phase diagram data published for the quinary system Al—Zn—Cu—Mg—Li. We have found that the level of reduction in solid solubility due to the lithium additions in decreasing order is copper, magnesium and finally, zinc.

[0022] Referring to Table I below, the broad, preferred and most preferred ranges for the main alloying elements will be set forth. TABLE I RANGE Zn Cu Mg Li Broad 5.0-13 1.0-3.0 1.0-6.0 .10-.99 Preferred 5.5-12 1.5-3.0 1.5-4.0 .10-.75 More Preferred 6.0-11 1.5-2.5 1.5-3.5  15-.50 Most Preferred 6.0-11 1.5-2.5 1.5-3.5 .4

[0023] The following example sets forth alloys and resulting wrought products made in accordance with the invention.

EXAMPLE

[0024] An ingot of an aluminum-zinc alloy having the following composition was cast: INGOT NO. 1 Si Fe Cu Mn Mg Zn Zr 0.02 0.01 2.08 0.01 1.89 6.41 0.11

[0025] Material fabricated from this ingot will be designated Alloy A hereinafter in this Example.

[0026] After this, the remaining molten metal was re-alloyed (i.e., alloying again an alloy already made) by adding 0.25% lithium to create a target addition of 0.25 wt % lithium. A second ingot was then cast having the following composition: INGOT NO. 2 Li Si Fe Cu Mn Mg Zn Zr 0.25 0.01 -0- 2.09 -0- 1.87 6.73 0.11

[0027] Material fabricated from this ingot will be designated Alloy B hereinafter in this Example.

[0028] Ingot No. 3 was created by re-alloying the remaining molten metal after casting Ingot No. 2 and then adding another 0.25 wt % lithium to create a total target addition of 0.50 wt % lithium. Ingot No. 3 had the following composition: INGOT NO. 3 Li Si Fe Cu Mn Mg Zn Zr 0.36 0.01 0.01 2.09 -0- 1.87 6.65 0.10

[0029] Material fabricated from this ingot will be designated Alloy C hereinafter in this Example.

[0030] Ingot No. 4 was created by re-alloying the remaining molten metal after casting Ingot No. 3 and then adding another 0.25 wt % lithium to create a total target addition of 0.75 wt % lithium. A fourth ingot was cast having the following composition: INGOT NO. 4 Li Si Fe Cu Mn Mg Zn Zr 0.62 0.02 0.01 2.00 -0- 1.80 6.97 0.10

[0031] Material fabricated from this ingot will be designated Alloy D hereinafter in this Example.

[0032] The four ingots were stress relieved and homogenized. The ingots were then subjected to a standard presoak treatment after which the ingots were machine scalped. The scalped ingots were then hot rolled into four (4) separate 0.7 inch gauge plates using hot rolling practices typical of 7XXX alloys.

[0033] After the four (4) separate plates were produced, a section of each of the plates was removed. Each of the four (4) sections were (a) solution heat treated; (b) quenched; and (c) stretched 1.5%. After this, eight (8) tensile strength test samples were produced from each of the treated four (4) sections, making a total of thirty-two (32) tensile strength test samples. One tensile strength test sample from each group of eight (8) (there being a total of four (4) plates in each group) was each subject to eight (8) different aging conditions, as described in the legend of FIG. 1. After this, tensile yield strength tests were performed, with the results being shown in FIG. 1. It will be seen that the alloys having lithium additions exhibited greater strength than those without lithium, while at the same time exhibiting thermal stability.

[0034] After this, the remainder of three of the four plates (i.e., Ingot No. 1 plate, Ingot No. 2 plate and Ingot No. 3 plate) was each cut into half to form pieces 1 and 2 for each plate, or a total of 6 pieces. Piece 1 of all three plates were (a) solution heat treated; (b) quenched; (c) stretched 1 ½%; and (d) aged to for 24 hours at 250° F. and then 6 hours at 350° F. These pieces were designated Alloy A-T6A; Alloy B-T6A; and Alloy C-T6A. Piece 2 of all three plates were (a) solution heat treated; (b) quenched; (c) stretched 1 ½%; and (d) aged for 24 hours at 250° F. and then 14 hours at 325° F. These pieces were designated Alloy A-T7B; Alloy B-T7B; and Alloy C-T7B.

[0035] Tensile and plane strain fracture toughness tests pursuant to ASTM E399 were then performed on these pieces. Also, resistance to exfoliation corrosion was evaluated using the ANCIT test. The ANCIT (aluminum-nitrate-chloride immersion test) has been developed as alternative to ASTM G34, the EXCO test, which is the most commonly accepted accelerated exfoliation corrosion test method. ANCIT includes an AlCl₃*6H₂O addition to the standard EXCO solution which buffers the starting pH from 0.3 to a value just above 3.0. This higher pH prevents the excessive pitting and intergranular corrosion that often occurs in EXCO and thus gives a more clear indication of exfoliation corrosion performance. See, S. Lee and B. W. Lifka, “Modification Of the EXCO Test Method For Exfoliation Corrosion Susceptibility In 7XXX, 2XXX and Aluminum-Lithium Alloys”, NEW METHODS FOR CORROSION TESTING OF ALUMINUM ALLOYS, ASTM STP 1134, V. S. Agarwala and G. M. Ugiansky, Eds., American Society for Testing and Materials, Philadelphia, 1992.

[0036] Referring now to FIG. 2, the specific strength, i.e., tensile yield strength divided by density for some of the pieces produced above is shown. It can be seen that improvements in the specific strength to density ratio were found for ancillary lithium additions.

[0037]FIG. 3 shows the strength/toughness relationship for the alloys from the current invention. It should be noted that as the strength increases by the lithium additions, the fracture toughness is decreased. This trade off between strength and toughness, which is a generally observed characteristic of aluminum alloys, is also apparent in data from alloy 7150 (designated by the filled triangles) from U.S. Pat. No. 5,108,520. 7150 is an aluminum-zinc alloy commonly used in aircraft construction and is representative of prior art. The dashed line is a linear fit to the 7150 data showing the trend of decreasing toughness with increasing strength. The solid line is a linear fit to the data from Alloys A, B and C. Invention Alloys B and C with ancillary Li additions exhibit significantly improved combinations of strength and fracture toughness with respect to Alloy A without ancillary additions and 7150 representing prior art.

[0038]FIG. 4 shows results from exfoliation corrosion testing. The ANCIT test was conducted. FIG. 4 shows that the ancillary lithium additions do not reduce the resistance to exfoliation. This is surprising because it would have been expected that corrosion resistance would have been reduced due to the higher affinity of lithium to form corrosion products.

[0039]FIGS. 5A and 5B show Dark Fields (DF) Transmission Electron Micrographs from samples from alloys A and C, respectively. These samples were aged to peak strength (T6A temper) . Note that the amount of precipitates is larger in the alloy with the lithium addition (Alloy C). In addition, the size of the precipitates is smaller for the alloy containing the ancillary lithium addition. This behavior was unexpected and is likely responsible for the higher strengths observed with lithium additions.

[0040] It will be appreciated that small amounts of lithium have not been conventionally added to aluminum base alloys since this typically leads to manufacturing difficulties such as cracking of plates during rolling or excessive formation of oxides during casting. Furthermore, lithium additions less than 1% would not provide a large reduction in density. Therefore, lithium additions below 1% had not been made. We found, however, that the small reduction in density coupled with an unexpectedly strong strengthening potential yields alloys with up to 21% higher specific strength. In addition, the plate products made from these alloys exhibited good fracture toughness and the corrosion resistance was not adversely affected. This was unexpected since lithium additions are known to reduce both corrosion resistance and fracture toughness.

[0041] While specific embodiments of the invention have been disclosed, it will be appreciated by those skilled in the art that various modifications and alterations to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular arrangements disclosed are meant to be illustrative only and not limiting as to the scope of the invention which is to be given the full breadth of the appended claims and any and all equivalents thereof. 

What is claimed is:
 1. An aluminum alloy comprising from about 5 to 13 wt % zinc and from about 0.10 to 0.99 wt % lithium.
 2. The aluminum alloy of claim 1, wherein said lithium content is from about 0.10 to 0.75 wt %.
 3. The aluminum alloy of claim 2, wherein said lithium content is from about 0.15 to 0.50 wt %.
 4. The aluminum alloy of claim 1, wherein said lithium content is about 0.36 wt %.
 5. The aluminum alloy of claim 1, wherein said zinc content is from about 5.5 to 12 wt %.
 6. The aluminum alloy of claim 5, wherein said zinc content is from about 6 to 11 wt %.
 7. The aluminum alloy of claim 1, including a dispersoid forming alloying element selected from the group consisting of chromium, vanadium, titanium and zirconium and mixtures thereof in the amount of from about 0.0 to 0.6 wt %.
 8. The aluminum alloy of claim 1, including a dispersoid forming alloying element selected from the group consisting of manganese, nickel, iron, hafnium, scandium and mixtures thereof in the amount of from about 0.0 to 1.0 wt %.
 9. The aluminum alloy of claim 1 that is a plate.
 10. The aluminum alloy of claim 1 that is an extrusion.
 11. The aluminum alloy of claim 1 that is a forged product.
 12. The aluminum alloy of claim 1 that is an aerospace structural component selected from the group consisting of a stringer, a wing spar and an upper wing section.
 13. The aluminum alloy of claim 1 that is a recreational product selected from the group consisting of a baseball bat, a softball bat, a shaft for an arrow, a golf club shaft and tubing for bicycles.
 14. The aluminum alloy of claim 1 having a yield strength (L) above about 80 ksi.
 15. The aluminum alloy of claim 14 having a yield strength (L) above about 85 ksi.
 16. The aluminum alloy of claim 1 having a fracture toughness of above about 25 ksi {square root}{square root over (inch)}.
 17. The aluminum alloy of claim 16 having a fracture toughness of above about 33 ksi {square root}{square root over (inch)}.
 18. The aluminum alloy of claim 17 having a fracture toughness of above about 35 ksi {square root}{square root over (inch)}.
 19. An aluminum alloy comprising from about 5 to 13 wt % zinc, from about 1 to 3 wt % copper, from about 1 to 6 wt % magnesium and from about 0.10 to 0.99 wt % lithium.
 20. The aluminum alloy of claim 19, wherein said lithium content is from about 0.10 to 0.75 wt %.
 21. The aluminum alloy of claim 20, wherein said lithium content is from about 0.15 to 0.50 wt %.
 22. The aluminum alloy of claim 16, wherein said lithium content is about 0.36 wt %.
 23. The aluminum alloy of claim 19, wherein said zinc content is from about 5.5 to 12 wt %.
 24. The aluminum alloy of claim 23, wherein said zinc content is from about 6 to 11 wt %.
 25. The aluminum alloy of claim 24, wherein said copper content is from about 1.5 to 3.0 wt %.
 26. The aluminum alloy of claim 25, wherein said copper content is from about 1.5 to 2.5 wt %.
 27. The aluminum alloy of claim 19, wherein said magnesium content is from about 1.5 to 4 wt %.
 28. The aluminum alloy of claim 27, including said magnesium content is from about 1.5 to 3.5 wt %.
 29. The aluminum alloy of claim 19, including a dispersoid forming alloying element selected from the group consisting of chromium, vanadium, titanium and zirconium and mixtures thereof in the amount of from about 0.0 to 0.6 wt %.
 30. The aluminum alloy of claim 19, including a dispersoid forming alloying element selected from the group consisting of manganese, nickel, iron, hafnium, scandium and mixtures thereof in the amount of from about 0.0 to 1.0 wt %.
 31. The aluminum alloy of claim 19 that is a plate.
 32. The aluminum alloy of claim 19 that is an extrusion.
 33. The aluminum alloy of claim 19 that is a forged product.
 34. The aluminum alloy of claim 19 that is an aerospace structural component selected from the group consisting of a stringer, a wing spar and an upper wing section.
 35. The aluminum alloy of claim 19 that is a recreational product selected from the group consisting of a baseball bat, a softball bat, a shaft for an arrow, a golf club shaft and tubing for bicycles.
 36. The aluminum alloy of claim 19 having a yield strength (L) above about 80 ksi.
 37. The aluminum alloy of claim 36 having a yield strength (L) above about 85 ksi.
 38. The aluminum alloy of claim 19 having a fracture toughness of above about 25 ksi {square root}{square root over (inch)}.
 39. The aluminum alloy of claim 38 having a fracture toughness of above about 33 ksi {square root}{square root over (inch)}.
 40. The aluminum alloy of claim 39 having a fracture toughness of above about 35 ksi {square root}{square root over (inch)}. 